The present invention relates to a batch type heat-treating method of heat-treating a large number of target substrates, e.g., semiconductor wafers, at once as in a vertical furnace or a lateral furnace.
Generally, as a processing apparatus for performing heat treatment, e.g., film deposition, oxidation, diffusion, or the like for a large number of semiconductor wafers at once, a batch type heat-treating apparatus such as a lateral furnace or a vertical furnace is known.
FIG. 11 is a schematic view showing the arrangement of a general batch type heat-treating apparatus. This batch type heat-treating apparatus 2 has a double tube structured process vessel 8 comprising an inner tube 4 and outer tube 6. A wafer boat 10 is accommodated in a process space S in the inner tube 4 from below. A large number of product wafers W, e.g., about 150 product wafers W, are placed on the wafer boat 10 in a full-loaded state at predetermined pitches, and are subjected to predetermined heat treatment, e.g., CVD (Chemical Vapor Deposition).
Various types of gases such as film deposition gases are introduced into the process vessel 8 from its lower portion, move upward in the process space S in the inner tube 4 while causing reaction, and are then turned downward and move downward through the space between the inner and outer tubes 4 and 6, to be exhausted to the outside. A heater (not shown) divided into zones is disposed around the process vessel 8. A process pressure is detected by a pressure sensor P1 provided at an exhaust port 12.
The process temperature, pressure, and gas flow rate employed when processing wafers fully loaded on the wafer boat 10 are obtained in accordance with the process purpose in advance. During the process, the process temperature, pressure, gas flow rate, and the like are controlled to match the process conditions. Generally, a plurality of side wafers (which do not become product wafers) for increasing the process reproducibility are placed at the upper and lower sides of the wafer boat 10. Monitor wafers (which do not become product wafers) for confirming the process result lie scattered in the product wafers.
Recently, various types of semiconductor devices are required, and accordingly various types of wafer-heat treatments for small-lot are sometimes needed. For example, when, e.g., 150 wafers must be processed as product wafers, the wafer boat 10 is fully loaded. Sometimes a smaller number of wafers than that, e.g., 100, 50, or 25 wafers, must be heat-treated.
In these cases, if an empty region where no wafer is set is present in the wafer boat 10, the wafer temperature and gas concentration may partially fall into disorder. Then, the wafer planar uniformity, surface-to-surface uniformity, and process rate of heat treatment change to decrease the heat-treating reproducibility, which is not preferable. In order to cope with this problem, the wafer boat 10 is fully loaded by using dummy wafers (which do not become product wafers) in number corresponding to the shortage, and heat treatment is performed with the ordinary process conditions for a wafer boat fully loaded with wafers. For example, 170 support grooves are formed in each support pillar 10A of the wafer boat 10 which can process 150 product wafers in a full-loaded state. Thirteen side wafers and 7 monitor wafers can be set respectively by the support grooves.
After a plurality of times of process operation, the dummy wafers are cleaned and are repeatedly used. Finally, the dummy wafers are discarded. Accordingly, use of dummy wafers increases the running cost.